class ActiveRecord::Relation::HashMerger

def other

the values.
build a relation to merge in rather than directly merging
interpolation might take place for where values. So we should
Applying values to a relation has some side effects. E.g. 
def other
  other = Relation.create(
    relation.model,
    table: relation.table,
    predicate_builder: relation.predicate_builder
  )
  hash.each do |k, v|
    k = :_select if k == :select
    if Array === v
      other.public_send("#{k}!", *v)
    else
      other.public_send("#{k}!", v)
    end
  end
  other
end